How do I know if my diastasis is closed? 

Closing the gap, is all about bringing the two halves of the rectus abdominal muscles together through exercise ( the elevators and contracting) done with the transverse brought all the way back towards the spine. You can bring the muscles even closer together by splinting. Doing the exercises 3 times a day will help ensure you are continually bringing your muscles closer together and bringing blood flow to the area.  Wearing the splint to approximate the muscles, making them easier to strengthen and will help take pressure off of the stretched out connective tissue ( the linea alba) so that it can heal.

As your gap heals, the connective tissue separation will feel shallower when you put your fingers in the gap to measure. As the connective tissue becomes shallower, the muscles will come closer together. 
Your diastasis is closed when the connective tissue at the belly button is shallow and the two muscle bellies are close together, maybe a finger width.  By clinical definition one to two fingerwidths and shallow is not significant, but I find my clients with two fingerwidths can still greatly benefit in both waistline and strength by bringing it closer together.